The present invention is directed to an improved airfoil for use with a helicopter rotor and more particularly to an airfoil section for at least partial use on a main rotor blade of a helicopter rotor.
Conventional rotary-wing aircraft have a forward airspeed limited by a number of factors. Among these is the tendency of the retreating blade to stall at high forward airspeeds. As the forward airspeed increases, the airflow velocity across the retreating blade slows such that the blade may approach a stall condition. In contrast, the airflow velocity across the advancing blade increases with increasing forward speed. Forward movement of the aircraft thereby generates a dissymmetry of lift between the advancing and retreating sides of the rotor. This dissymmetry may create an unstable condition if lift is not equalized across the advancing and retreating sides of the rotor.
At higher speeds, such as, at the outer tip regions of conventional rotary wing aircraft or on high-speed rotary wing aircrafts, for example, on a helicopter including a coaxial contra-rotating rotor system with an auxiliary translational propulsion system, high performance 2-dimensional airfoils are required. That is, the requirements on an airfoil section at higher speeds are generally more complex than those for a fixed wing aircraft because, on a single revolution of the rotor blade, an airfoil section thereof experiences lift coefficients from negative values to positive value with section Mach numbers from subsonic to transonic values. Since the ranges of lift coefficients and Mach numbers experienced by an airfoil section depend on its radial location along the rotor blade and the rotary-wing aircraft flight conditions, different airfoil sections have been used for a specified range of radial positions along the rotor blade span.
The maximum lift coefficient of an airfoil section is of considerable importance in the process of selecting airfoils for application to a rotary-wing aircraft rotor. When the maximum lift coefficient of an airfoil section is exceeded (i.e., the airfoil is stalled), the corresponding drag coefficient increases dramatically and the pitching moment coefficient can change direction (nose-up to nose-down) as well as change greatly in magnitude. When a significant part of the rotor blade is operating beyond the maximum lift coefficient of the local airfoil section, the power required to sustain flight exceeds the power available, thus limiting the particular flight condition. This could occur with increases in aircraft gross weight, in maneuvers, or in forward flight.
The problem may occur over the outer portion of a rotor blade such as that utilized on high-speed rotary wing aircraft or at the outer tip regions of conventional rotary wing aircraft rotor blades where drag divergence Mach numbers of present state-of-the art rotary wing airfoils are exceeded. Reducing airfoil thickness ratios can alleviate this problem, however, typically at the expense of hover figure of merit due to the decrease in an airfoil's maximum lift capability when thickness is reduced.
Hover figure of merit can then be increased with additional blade area, however, this approach reduces cruise efficiency since the increased blade area is not required in this flight regime. Some compromise between hover figure of merit and cruise efficiency must thereby be achieved by the designer. Designing airfoils with higher maximum lift coefficients at a desired Mach number while simultaneously increasing the drag divergence Mach number for moderate lift coefficients provides the rotor designer the additional aerodynamic performance necessary to get cruise speeds up into the 400 knot range.
As manufacturers look at new ways to increase the speed of rotary wing aircraft significantly different rotor blade designs are required. Accordingly, it is desirable to provide an improved airfoil family for contra-rotating rotor systems as well as outboard stations of single rotor rotary-wing aircraft.